1. Field of the Invention
The present invention relates to a heat fixing apparatus, and more particularly, to a heat fixing apparatus in an image forming apparatus using an imaging process such as an electrophotographic process and an electrostatic recording process. Further, more specifically, the present invention relates to an image forming apparatus having a heat fixing apparatus which performs a heat fixing process on an unfixed toner image of target image information formed and carried on a recording material, as a fixed image, in a transfer process or a direct process by the imaging process portion. Here, the examples of the recording material include a transfer material, a print paper, a photosensitive paper, and an electrostatic recording paper.
2. Description of the Related Art
An apparatus using an endless belt (endless film) has been in practical use as a heat fixing apparatus. A typical example is illustrated in FIG. 13. More specifically, in FIG. 13, a film assembly 60 includes a heating heater 61, a stay holder 62, and a thin film (fixing film) 63. The heating heater 61 has a heat generation resistance layer generating heat by electrification, the heat generation resistance layer being formed on a ceramic substrate made of alumina, aluminum nitride or the like. The heating heater 61 is fixed to the stay holder 62 made of heat-resistant resin. The heat-resistant thin film (hereinafter referred to as fixing film) 63 is made of a resin such as a polyimide or a metal such as Stainless Used Steel (SUS) and is loosely fit on the stay holder 62. The fixing film 63 is pressure-sandwiched between the heating heater 61 of the film assembly 60 and an elastic pressure roller 50 to form a fixing nip portion.
The elastic pressure roller 50 includes a metal core 51, an elastic layer 52 made of silicon rubber or the like and provided on the outer surface of the metal core 51, and a mold release layer 53 made of a fluorocarbon resin or the like. The fixing film 63 is transported and moved in a direction of the arrow sliding in close contact with the heating heater 61 at the fixing nip portion by a rotational drive force of the elastic pressure roller 50 in the direction of the arrow. The temperature of the heating heater 61 is detected by a temperature detection unit 64 such as a thermistor provided on the rear surface of the heater, is fed back to a power control portion (not shown), and the heating heater 61 is adjusted to be at a predetermined constant temperature (fixing temperature). Various image forming apparatuses as a printer and a copy machine having such a heat fixing apparatus using a film heating process have a lot of advantages in comparison with a conventional heat fixing apparatus using a heat roller process. The examples of the advantages include a high heating efficiency and a quick activation, which can eliminate the necessity of preheating during a wait time and can reduce the wait time.
Recently, various types of print media (recording material) have been used for copy machines and printers. In order to handle such a diversity of media types, the heat fixing apparatus also needs to adjust the fixing conditions for the specific medium.
As one of the means for changing the fixing conditions, there has been a method of changing the pressure applied to the fixing nip portion. For example, the Japanese Patent Application Laid-Open No. 2007-128037 discloses a measure by which when an envelope is printed, the pressure applied to the fixing nip portion is made lower than when a regular paper is printed to prevent the envelope from been deflected.
However, when the recording material is exposed to a reduced pressurization condition (second pressurization condition), the following problems occur.
More specifically, when pressure is applied to a pressurization member and a heating member at both lengthwise edge portions perpendicular to the conveyance direction of the recording material, the pressurization member and the heating member are deflected. The greater the applied pressure, the greater the deflection. When the pressurization member and the heating member suffer a large amount of deflection due to their light weight and use of low cost materials, at least one of the pressurization member and the heating member needs to be formed in a crown shape (the center portion is larger than the edge portions) allowing for the amount of deflection. This allows an optimal nip shape to be set under the normal pressurization condition (first pressurization condition).
For this reason, when the pressure is applied to the fixing nip portion as the second pressurization condition in order to prevent the envelope from being deflected, the amount of deflection is reduced accordingly. However, the width of the fixing nip portion becomes uneven in the longitudinal direction. More specifically, the width of the fixing nip portion in the edge portions becomes smaller than that in the center portion.
Therefore, when a sheet is passed (fixing process) under the second pressurization with the same heat distribution (in applied current proportion of a respective heat generation member) of heat generation members as at the first pressurization, the heat of the heating member is difficult to be transmitted to the pressurization member in the edge portions because the width of the fixing nip portion in the edge portions is small (narrow). Therefore, the temperature of the heating member excessively rises, thereby causing a problem in that the durability of the fixing member is reduced.
Further, when the temperature of the fixing apparatus is raised with the pressure applied to the fixing nip portion corresponding to the second pressurization condition, the heat of the heating member is difficult to be transmitted to the pressurization member in the edge portions, thereby causing the same problem as the fixing process under a reduced pressurization condition.